Since their discovery, polyamides have become a class of materials of increasing importance. They are now available in a multitude of modified forms and, in particular, as mixtures with various other polymers for a variety of uses. In order to enhance their usefulness, attempts have been made to integrate the advantageous properties of polyamides with those of polyurethanes by simple physical mixture of the respective polymers in the molten state. However, mixtures of this type generally exhibit poorer properties than the individual components because of the limited compatibility of the polyamides with polyurethanes.
In theory, another way in which to combine the properties of polyamides and polyurethanes in one product is by the construction of suitable block polymers in which alternating polyurethane and polyamide segments are connected with each other by chemical bonds. DE-AS-1,224,031 (BASF) gives examples in which polyether-polyol groups can be linked with low molecular polyamides carrying carboxyl or amino end groups via urethane and amide or urea groups. To this end, the polyether-polyol is first reacted with a molar excess (up to 2 mol) of diisocyanates and the isocyanate-modified polyether-polyol is reacted with the carboxyl-amino groups of the polyamides.
These products can be used, in small amounts, as anti-static additives for thermoplasts, especially polyamides. They are not suitable for thersoplastic processing; e.g. for injection-molded and extruded bodies. In the reaction of diisocyanates with polyamides which carry amino, hydroxyl or carboxyl terminal groups, there are usually side-reactions which very quickly lead to partial cross-linking of the resultant polymers of the type described in DE-AS-1,224,031, especially if these polymers are prepared at high temperatures in the melt. The cross-linking reactions which take place, e.g. as a result of the reaction of isocyanate with amide groups (which is difficult to control), can proceed so far that the polymers which are obtained block conventional injection molding machines, extruders, blowing and foil apparatus, because they can no longer be melted and worked. Therefore, they are wholly unsuitable for present-day thermoplast processing. Owing to their poor meltability and resultant poor flowability, polyamides modified with diisocyanates in accordance with DE-AS-1,224,031 also cannot be used as hot melt adhesives.
Also, in other linking reactions of polyamides with isocyanate-terminated polyurethanes, it is impossible to avoid the cross-linking which immediately ensues, and which leads to similarly unusable products. Thus, the object of the present invention is a process for the preparation of such thermoplastic processable block polymers with amide and urethane segments, which no longer exhibit the disadvantage of comparable products previously known.